I have a problem with a transistor but I don't understand why. Using a 2N22222A transistor to drive a dc motor, after having made all the connections correctly (emitter to ground, collector and positive of the external power supply to the motor, the base with arduino and a 330 ohm resistor) and having connected the GND of the external power supply (3.7 volt battery) to arduino, the motor actually works, but the revolutions per minute are far inferior and what it should do without the transistor, I don't understand why. PS the components are all working.
When you use a transistor you loose at least 0.7V of the voltage across the transistor. So your motor is only getting just 3V. This might not be enough. It is quite OK to use a supply of 5V.
The other thing is that the resistor could be lower, I would suggest 130 ohms or just over. This will give the base more current an allow the transistor to switch a higher current load.
Kk many thanks. About the resistance in this case being 130 a very low value, isn't it better not to put it just so as to receive more current? i checked that the 2N2222A support up to 6v on the base
Arduino output can only supply 20-30mA into the base.
Smallest base resistor should thus be (Vout - Vbe)/30mA = 143 ohm at 5V and 0.7V Vbe.
130ohm would result in 33mA, which would still be okay.
Note that the output is only guaranteed to be 4.2V with a 20mA load; a 33mA load will drag the output down more. You'll have to experiment to measure voltage across the resistor to determine actual current flow.
For better performance, replace the NPN with an N-channel MOSFET. They can have on-resistance of just hundreths of an ohm (like 0.05 ohm), so more voltage is dropped across the motor for more current flow and higher speed.
Ok many thanks guys
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